The present invention relates to an internal combustion engine having a two-spark-plug type auxiliary combustion chamber, which is designed to perform selective ignition depending on the operating condition of the engine.
To burn a lean air-fuel mixture is one of the effective measures to decrease such toxic substances as hydrocarbon, carbon monoxide and nitrogen oxides that are usually present in the exhaust gases emanating from an internal combustion engine. Although that lean mixture is ignitable and combustible, it causes the flame produced to propagate slowly, thereby seriously decreasing the thermal efficiency of the engine cycle. The propagation velocity of the flame depends on the amount of the energy of its ignition source and the degree of turbulence in the air-fuel mixture burned. Namely, the greater the ignition energy, the greater will be the speed of the flame propagation. Hence, there has been already proposed a method to burn the lean mixture in the main combustion chamber with the flame projected from an auxiliary combustion chamber provided adjacent thereto.
There are several types of internal combustions engines that have such an auxiliary combustion chamber. In one of them, an intake valve, to draw in a comparatively richer air-fuel mixture, is provided in the auxiliary combustion chamber, in order to make the ignition therein easier. This type of internal combustion engine equipped with the auxiliary combustion chamber having such intake valve is reliable in terms of the ease and certainty with which the mixture is ignited. However, it tends to become complex in structure, since it has to contain in itself actuating mechanisms not only for the intake and exhaust valves in the main combustion chamber, but also for the intake valve to draw the richer mixture into the auxiliary combustion chamber. Also, an internal combustion engine that has no intake valve in its auxiliary combustion chamber has been proposed. In this latter type of engine, a lean air-fuel mixture is urged from the main combustion chamber into the auxiliary combustion chamber, during the compression stroke of the engine, so that it is ignited therein. This type of engine is disadvantageous because the exhaust gas of the preceding stroke that might be remaining in the auxiliary combustion chamber impedes the ignition of the fresh mixture therein. This disadvantage can however be easily overcome by providing some means that perfectly scavenges such residual gas out of the auxiliary combustion chamber so that the engine becomes very useful, is simple in structure, and reliable in operation.
In either type of the aforesaid internal combustion engines having an auxiliary combustion chamber, a spark plug is fitted in the auxiliary combustion chamber, so that the air-fuel mixture is ignited and burned therein.
More particularly, the spark plug is provided at a suitable position in the auxiliary combustion chamber so that its electrodes are disposed, for instance, in the vicinity of the opening of the auxiliary combustion chamber through which it communicates with the main combustion chamber, thus making it possible to always supply a fresh mixture to the electrodes. This also helps to prevent the spark plug from becoming contaminated. On the other hand, however, when the piston reciprocates rapidly during high-speed driving, the mixture in the main combustion chamber flows into the auxiliary chamber at such a great velocity that a violent turbulence is likely to occur in the latter. On such occasions, the nucleus of the flame formed around the electrodes of the spark plug is blown out before it grows to the size necessary for perfect ignition unless ignition timing is controlled very accurately. Also, when starting the engine at low temperature, the combustibleair-fuel mixture does not readily condense in the vicinity of the spark plug's electrodes because the spark plug is located apart from the main combustion chamber.
This fact is responsible for the rather poor starting performance of this type of engine. In addition, the electrodes of the spark plug, being exposed to the strong projected flame, are liable to melt away, which in turn sometimes causes preignition.